This invention relates to a cone penetrometer system for conducting a Cone Penetrometer Test (CPT). In particular, the invention is directed to an apparatus for lubricating the push rod portion of a cone penetrometer upstream of the instrumentation package to thereby improve measurement accuracy and test depth without damaging the soil.
The so-called Cone Penetrometer Test (CPT) has been used to classify soils and characterize sites in various applications including, for example, dam construction and maintenance and other major construction projects. See for example, "Site Characterization Using the Cone Penetrometer Test", Olsen et al., ASCE Specialty Conference, Use of In-Situ Testing in Geothechnical Engineering, Jun. 22-25, 1986 and "Soil Classification and Site Characterization Using the Cone Penetrometer Test", Olsen et al., First International Symposium on Penetrometer Testing, Mar. 20-24, 1988. The standard equipment and methodology is described in "Standard Test Method for Deep Quasi-Static, Cone and Friction-Cone Penetration Tests of Soil" ASTM D 3441-86.
In General, the cone penetrometer test data may be analyzed to provide information on soil strength and soil classification. CPT data can be normalized with respect to vertical effective stress for comparison of data from various depths in soil.
The typical CPT test, as performed in the United States, is illustrated in FIG. 1 and generally comprises pushing a known 3.57 centimeter diameter electrical CPT probe 10 into the earth 11 at 2 cm/sec using one or more interconnected 1 meter hollow push rods 12 in a string 13 driven with the reaction force of a large mass such as a truck 14. The probe 10 includes an instrument housing 16 at the distal end 18 of the push rod string 13. A conical tip 20, connected to the housing 16, penetrates the earth. A tubular section of the penetrometer, called the friction sleeve 21, is located directly above the cone tip 20, but it is physically separated from it. A cone support 23 connects the tip 20 to the instrument housing 16. The friction sleeve 21 surrounds the cone support and carries one or more sensors, e.g. strain gauges 25, forming a load cell 27 there-in. The sensors measure local side friction resistance developed between the friction sleeve 21 and the surrounding soil 11 independently of the force exerted on the tip 20. Devices (not shown) in the instrumentation package 16 are responsive to the sensors and are coupled to equipment in the truck 14 by means of wires 22 which run up to the surface inside the string 13 of the hollow push rods 12.
Generally two measurements are recorded, namely, cone resistance q.sub.c which is an end bearing stress and sleeve friction resistance f.sub.s, which is a localized large-strain index of sheer strength. Both measurements are usually reported in terms of tons per square foot (tsf) although metric units may also be used if desired.
In the past, the metal penetrometer rod was forced into the soil with no lubrication. The depth of penetration is determined by the point in which the point penetration resistance and the total side wall friction resistance equal the weight of the reaction mass (typically 20 tons) being used to drive the penetrometer into the ground. This system is inefficient because the total side wall friction on the rod sections 12 in the ground 11 quickly limits the depth of penetration even when the tip resistance is low. Some penetrometers have been built with ports for ejecting drilling mud 32 into the area between the string 13 and the soil 11. However, these units are complicated and cumbersome to use because of the problems associated with the handling of drilling mud and the mud injection system. More importantly, however, drilling mud under pressure can cause damage or fracture 34 the soil 11. Fractures 34 are unacceptable, especially if a test is performed at a dam site where such damage may propagate. Finally, drilling mud is dangerous and difficult to clean up because it creates slippery surfaces.